1. Field of the Invention
The subject invention relates to a system and method for determining a voltage in each cell of a plurality of battery cells connected in series.
2. Description of the Related Art
Electric vehicles and hybrid-electric vehicles typically utilize numerous cells (i.e., batteries) for powering electric drive motors and other electric equipment. These cells are often connected together in a series relationship, as is well known to those skilled in the art, to provide higher voltages.
Due to variations between individual cells, such series-connected cells require periodic balancing, i.e., charge equalization, to maintain a steady voltage and prevent premature failure. One difficulty in cell balancing is determining which cell or cells may need to individually charged or replaced. Therefore, it is necessary to provide a system to determine the voltage in each cell.
Numerous systems and techniques have been developed to address this necessity. One such system is disclosed in U.S. Pat. No. 5,914,606 (the '606 patent) to Becker-Irvin. The system disclosed in the '606 patent teaches each individual cell being directly connected to an amplifier through a bank of switches. A controller is also connected to the amplifier to measure the voltage of the selected cell. Unfortunately, the system of the '606 patent does not provide any isolation between the cells and the controller, resulting in a fluctuating voltage, especially when the cells are in use. Furthermore, the system of the '606 patent would require high precision resistor dividers to divide the voltage down to an acceptable level for measurement.
Other systems utilize one or more capacitors between the cells and the amplifier such that a more stable and accurate voltage may be measured. For instance, U.S. Pat. No. 6,362,627 (the '627 patent) to Shimamoto et al. discloses a system with a plurality of cell switches connected to various cells, a capacitor connected to the cell switches, and an amplifier connected to the capacitor. The '627 patent uses a secondary polarity correction, which would add to measurement error.
Japanese Patent Abstract Publication No. 2003240806 (the '806 publication) to Yazaki Corporation discloses a system for measuring voltage of individual cells in a plurality of cells connected in series. The system includes a plurality of cell switches electrically connected to the plurality of cells with one switch connected on each side of each cell. A first bus is electrically connected to the second side of every other cell switch and a second bus is electrically connected to the second side of each cell switch not connected to the first bus. A capacitor is electrically connected between the busses. Four bus switches are electrically connected to the capacitor and the busses: one pair of bus switches allow connection between either side of the capacitor and an amplifier, while another pair of bus switches allows connection between either side of the capacitor and ground. The system of the '806 publication may not accommodate an implementation with MOSFET-type transistors as switches, since some current must be drawn from the system in order to turn on the MOSFETs.
Despite the various systems described above and existing elsewhere in the prior art, there remains an opportunity for a system for measuring voltage of individual cells connected in series having improved accuracy, higher measurement speed, and lower implementation costs.